1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, facsimile or printer including a fusing unit to fuse toner to a printing paper, and provides an improved image forming apparatus with respect to temperature control in STANDBY mode of a heater.
2. Description of the Related Art
An image forming apparatus including STANDBY mode shifts an operation mode from PRINT mode to STANDBY mode when a predetermined period of time passes since a printing operation is finished. With the image forming apparatus, an upper limit temperature and a lower limit temperature in STANDBY mode are previously set with regard to a surface temperature of a heat roller. When the surface temperature reaches the upper limit temperature, the image forming apparatus stops supplying heat to the heat roller by turning off a heater, and also when the surface temperature reaches the lower limit temperature, the image forming apparatus resumes supplying heat to the heat roller by turning on the heater. However, this temperature control cannot prevent temperature overshoot in which the surface temperature of the heat roller exceeds the upper limit temperature and temperature undershoot in which the surface temperature of the heat roller falls below the lower limit temperature. In particular, where an image forming apparatus uses a heat roller with a lower thermal capacity due to a thin tube in order to shorten a warming-up time, temperature overshoot or undershoot is likely to occur.
In order to prevent the temperature overshoot or undershoot, the conventional image forming apparatus turns off the heater at a lower temperature than the upper limit temperature, provided that the surface temperature of the heat roller is rising. Likewise, conventional image forming apparatus turns on the heater at a higher temperature than the lower limit temperature, provided that the surface temperature of the heat roller is falling. In other words, the conventional image forming apparatus turns on/off the heater in the earlier stage of fusing so as to prevent the heat roller from causing temperature overshoot or undershoot.
Further, the conventional image forming apparatus gradually lowers the surface temperature of the heat roller. The conventional image forming apparatus lowers a surface temperature setting of the heat roller gradually every time a predetermined period of time, for example, 10 seconds, passes since the surface temperature thereof reaches a fusing temperature, and is configured so that power consumption by a heat source can be reduced while maintaining the surface temperature of the heat roller within a range of temperatures suitable for fusing. Furthermore, the temperature settings are gradually lowered according to a heat radiation rate of the heat roller.
According to the conventional image forming apparatus, temperature overshoot or undershoot can be avoided to some extent by turning on/off a heater at the early stage. In this method, however, only a difference between a transitory value of a roller temperature and a target temperature becomes smaller, which cannot completely prevent temperature overshoot or undershoot.
According to the conventional image forming apparatus, unless the surface temperature of the heat roller falls to a lower fusing limit temperature after the surface temperature of the heat roller reaches the fusing temperature, power consumption by a heat source can be reduced by gradually lowering the temperature setting. However, because the difference between the fusing upper limit temperature and the fusing lower limit temperature is subtle, the image forming apparatus reduces only a small amount of power, which provides less contribution to power saving.
Although a higher temperature in STANDBY mode smoothly shifts an operation mode to PRINT mode, an increase in power consumption by a heat source is inevitable. Further, although a greater difference between the upper limit temperature and the lower limit temperature in STANDBY mode can reduce power consumption by the heat source, shifting to PRINT mode may require more time. By setting a standby temperature at an intermediate temperature between the upper limit temperature and the lower limit temperature in STANDBY mode, the time taken to shift the operation mode to PRINT mode can be shortened while power consumption by the heat source is reduced to some extent. However, in this case, stuffy heat in the surrounding area of the fusing unit is inevitable, and even in STANDBY mode, the stuffy heat needs to be expelled from a case by driving a cooling fan, which inevitably increases power consumption.